The interpretation of lidar data on fog has been limited by two obstacles: approximations in the form of the Mie scattering cross sections for water droplets, and droplet size distribution functions whose relationship to the experiment has not been clear. This paper develops a method for generating distribution functions from experimental data. These functions are then used with newly available Mie cross sections to obtain backscattering and extinction coefficients for singly scattered ruby laser pulses in fog. The results show what experimental lidar accuracies are needed to uniquely determine fog droplet size distribution.